1. Field of the Invention
This invention relates to an optical component for use in two-way optical communication in which transmission and reception of light are effected through a single optical fiber, and an optical transmitter-receiver constructed by the use of the optical component.
2. Description of the Related Art
FIGS. 6A and 6B illustrate a previous optical component (which will be referred to as previous device hereinafter) of the type concerned which has been devised in the factory facilities of the assignee of the present patent application, although not publicly known and from which the present invention originated, and an optical transmitter-receiver constructed by incorporating the component therein together with an optical fiber and light-receiving and light-emitting elements. This previous device will be described below as an example of comparison merely for facilitating of the understanding of the present invention.
In the previous device illustrated, the optical component 10 is constructed by using a prism 11 of pentagular shape (five-sided polygon) in cross-section having first to fifth operative faces 11a-11e which take part in optical transmission and reception. These first to fifth operative faces 11a-11e are all formed perpendicular to the plane of the drawing of FIG. 6A and have respective opposite ends perpendicular to the plane of the drawing. It should be noted, of course, that the prism has end faces 11g and 11h parallel to the plane of the drawing (see FIG. 6B).
One end of the first face 11a of the prism 11 and one end of the second face 11b adjoin each other at a corner Pab such these faces 11a and 11b form a right angle and similarly the other end of the first face 11a and one end of the third face 11c adjoin each other at a corner Pac such these faces 11a and 11c form a right angle. It is thus to be understood that the second face 11b and the third face 11c oppose each other in parallelism.
The other end of the second face 11b of the prism 11 and one end of the fourth face 11d adjoin each other at a corner Pbd, and the other end of the third face 11c and one end of the fifth face 11e adjoin each other at a corner Pce.
Further, the other end of the fourth face 11d and the other end of the fifth face 11e adjoin each other at a corner P, and the fourth face 11d and the fifth face 11e are recessed inwardly from the corners Pbd and Pce toward the first face 11a and form a V-shape as viewed in FIG. 6A. That is, the corner P at which the fourth face 11d and the fifth face le adjoin each other is located adjacent the first face 11a closer than the corners Pbd and Pce are.
The first face 11a and the second face 11b of the prism 11 are formed integrally with first and second condensing lens 12 and 13, respectively for light-to-be-received, and further the first face 11a and the third face 11c are formed integrally with first and second condensing lens 14 and 15, respectively for light-to-be-transmitted. In addition, it is to be noted that the first light-to-be-received condensing lens 12 and the first light-to-be-transmitted condensing lens 14 provided on the first face 11a are partially cut away at planes perpendicular to the plane of the drawing such that the cut surfaces of those lens are joined together.
An optical fiber 21 has an end face 21b at its one end located adjacent and in opposition to the lens 12 and 14 formed on the face 11a and has its axis 21a coincide at a point of intersection between a first plane X (shown in a one-dotted chain line in FIG. 6B) passing through the coupled end surfaces (interface) between the first light-to-be-received condensing lens 12 and the first light-to-be-transmitted condensing lens 14 and the corner P and perpendicular to the plane of the drawing of FIG. 6A on one hand and a second plane Y (shown in a one-dotted chain line in FIG. 6B) passing through the centers of the lens 12, 13, 14 and 15 parallel to the plane of the drawing of FIG. 6A and orthogonal to the first plane X on the other hand.
With this arrangement, the upper half portion of the prism 11 located above the first plane X constitutes a receiving path while the lower half portion of the prism 11 located below the first plane X constitutes a transmitting path.
A light-receiving element 22 is positioned in opposition to the lens 13 on the face 11b with its center A22 aligned with the central axis A13 of the lens 13 while a light-emitting element 23 is positioned in opposition to the lens 15 on the face 11c with its center A23 aligned with the central axis A15 of the lens 15. It is also to be noted that the light-receiving element 22 and the light-emitting element 23 are oppositely positioned in parallel.
In this example, the light-receiving element 22 and the light-emitting element 23 are both mounted on a reed frame 24 and resin-encapsulated in transparent resin. In the drawing, 24 indicates the reed frame and 25 an encapsulating resin. This encapsulating resin has a lens portion 25a. The light-emitting element 23 may be a laser diode (LD) or a light-emitting diode (LED), for example, and the light-receiving element 22 may be a photodiode (PD), for example.
As shown in FIG. 6A, a light 31 to be received which has been emitted from the end face 21b of the optical fiber 21 is condensed through the light-to-be-received condensing lens 12 prior to entering the prism 11 through the face 11a and is then reflected by the face 11d to be directed at the face 11b, followed by being condensed through the light-to-be-received condensing lens 13 before entering the light-receiving element 22. In this regard, the lens portion 25a of encapsulating resin aids the light-receiving element in condensing the light.
On the other hand, a light 32 to be transmitted which has been emitted from the light-emitting element 23 is condensed by the light-to-be-transmitted condensing lens 15 prior to entering the prism 11 through the face 11c and is then reflected by the face le to be directed at the face 11a, followed by being condensed through the light-to-be-transmitted condensing lens 14 before entering the end face 21b of the optical fiber 21.
It is thus to be appreciated that in the illustrated example the arrangement is such that transmission and reception of light is effected through a single optical component 10.
It should be here appreciated that in this type of optical component in charge of both transmission and reception of light, crosstalk is a great concern with respect to its performance and that it is a significant problem to suppress the crosstalk.
Crosstalk in the optical component of the type concerned means that light being transmitted leaks into the receiving side in this side station and enters a light-receiving element in this side station. Particularly, crosstalk ascribable to reflection at an optical interface in the parting area between the receiving path and the transmitting path in this side station or at a proximal end face of the optical fiber is called near-end crosstalk.
In contrast, crosstalk ascribable to reflection at an optical interface in the parting area between the receiving path and the transmitting path in the other side (opponent's) station or at a distal end face of the optical fiber or at the faces of a light-receiving element and light-emitting element in the other side station is called far-end crosstalk. The optical component 10 shown in FIGS. 6A and 6B and the optical transmitter-receiver constructed by combining the optical component 10 with the light-receiving element 22 and the light-emitting element 23 in the illustrated arrangement has been found to have the construction apt to cause especially far-end crosstalk. That is, the previous device described above cannot avoid causing crosstalk. More specifically, it has been found as a result of researching into the cause of such crosstalk that as shown in FIG. 7, since the tilt angles α1 and α2 of the faces (reflective faces) 11d and 11e, respectively relative to the plane X passing through the corner P and the axis 21a of the optical fiber 21 and perpendicular to the plane of the drawing are both set to be relatively large, say at 45°, the light 31 to be received may enter the respective faces 22a and 23a of the light-receiving element 22 and the light-emitting element 23 and the light reflected by these element faces may again follow the path along which it has entered before back into the optical fiber 21 so that it is apt to cause far-end crosstalk in the other side station.
In addition, since the light-receiving element 22 and the light-emitting element 23 are both positioned in direct opposition to the lens 13 and 15, respectively and the centers A22 and A23 of those elements are aligned with the central axes 13a and 15a of the lens 13 and 15, respectively, the previous device are constructed in this respect as well such that the light reflected by these element faces may again follow the path along which it has entered before back into the optical fiber 21, tending to cause far-end crosstalk in the other side station.